fix: pass ContentType to COS putObject and ensure charset=utf-8 for text uploads

[Jerry-Xin]

Summary

Bot file uploads to COS were missing the ContentType parameter in putObject, causing COS to default to application/octet-stream. This made text files (e.g. .md) download instead of displaying inline in the browser, and text content appeared garbled due to missing charset.

Changes

src/api-fetch.ts

• Add ContentType to COS putObject params (the core fix)
• Add ensureTextCharset() helper: appends ; charset=utf-8 to text/* types lacking charset
• Expand inferContentType() map with text file extensions: .md, .markdown, .csv, .html, .htm, .css, .xml, .yaml, .yml

src/channel.ts

• When download returns application/octet-stream, fall back to inferContentType(filename) to get the real type
• Wrap contentType with ensureTextCharset() before upload

src/inbound.ts

• Same ensureTextCharset() wrapping for inbound file re-upload path

Tests

• 8 tests for ensureTextCharset covering text types, non-text passthrough, and existing charset preservation

Closes #137